This present disclosure is directed to a spark discharge system which forms photons which are emitted by a pulsed spark discharge across a pair of terminals. The spectra are created by spark interaction with a flow of an inert gas, helium being the preferred gas stream, in the region of the pulsed spark discharge. This creates the photon ionization for use with the sample which will be described.
The improved system of this disclosure utilizes a helium flow through the region of the spark discharge. This helium flow is the only flowing material in the immediate region of the spark. An instrument system is constructed with this so that the helium flow continues through a central axial passage of the equipment to a region having first, second and third ring shaped electrodes surrounding the passage way. The flow is directed to the three ring shaped electrodes so that the flow is able to interact with a sample gas and dopant which is delivered into the flowing helium gas. More specifically, an electron capture detector (ECD hereinafter) is provided by incorporating a trace or dopant gas which is injected into the flow through the three electrodes. The trace gas interacts with the photon ionization emission from the spark to create a charged gas flow. This establishes a current flow between two of the electrodes which current flow can be detected by an electrometer connected across the two electrodes. This current flow defines the base line signal in the equipment. The baseline is adjusted by changing the flow rate of the dopant, by changing the point at which the dopant is introduced, or by also changing or adjusting the position at which the dopant is introduced with respect to the circular electrodes. The baseline condition established is a maximum current flow.
As a test instrument, the ECD utilizes a flow of gas discharged from a gas chromatograph (GC) column or other suitable source. The GC column effluent normally carries with it a carrier gas which is routinely present at a specified flow rate. In addition to that, the GC carrier gas delivers in timed sequence peaks of constituents in a tested sample. For instance, in testing the output of any typical petrochemical product manufactured in large volume, analysis of the purity of the produced product is desirable. One mode of testing is to utilize the GC column which elutes the various constituents in a specific timed sequence dependent on the relative mobility as the sample constituents travel through the GC column. A typical GC column comprises a mobile phase and a stationary phase. The mobile phase comprises a carrier gas such as helium into which sample gas containing one or more compounds are injected. The stationary phase comprises one or more solid constituents within the GC column which exhibit different retention times for the "unknown" sample compounds. The sample gas containing the unknown compounds is injected over a relatively short period of time into the carrier gas flow near the input of the GC column. Sample compounds are retained for different times by the stationary element of the GC, and then subsequently released. Upon release, each type of sample compound is swept by the carrier gas from the GC column and discharged in the form of a "peak" or mixima in concentration in the carrier gas. Retention times, and therefore time separation of the unknown compound peaks, is a function of several factors including the carrier gas flow rate and the type of the stationary phase within the GC column. The result of an injection near the input of a sample gas containing multiple compounds results in the subsequent release, or "eluates", of maxima or peak concentrations of individual compounds at the output of the GC column. Again, time recording of the GC output reflects these elutes as peaks. Stated another way, the GC separates sample compounds by eluting in the form of concentration maxima or peaks in the output carrier gas at varying times, measured from the injection of the composite sample gas. As described, the GC process does not quantify the concentrations of the unknown compounds, but does separate multiple compounds for further analysis using the current ECD invention. By using a series of calibration gases, a fixed flow rate, and a specific fixed phase material, the GC process can be used to identify compound types based upon the time position of the eluted peaks, measured with respect to the injection of the composite gaseous sample. There may be any number of eluted peaks formed by the GC column output which peaks must be detected and quantified. The ECD system is a good technique for peak quantification. Enhanced sensitivity is therefore obtained as the peaks of the sample are passed through the ECD device.
Adjustment of an ECD is somewhat delicate. The present disclosure sets forth an arrangement which can be readily adjusted. In this particular version, the ECD forms a baseline current as a result of photon ionization of helium flowing through the pulsed spark discharge. That creates radiation sufficient to interact with an introduced dopant located strictly in an isolated region downstream from the spark creating electrodes. The electrodes forming the spark are isolated in an atmosphere of helium and therefore have an extended life. Dopant materials introduced elsewhere in the system are introduced at such a low flow rate that they are swept away from the pulsed spark terminals. Moreover, this assures that the pulsed discharge interacts only with the inert helium, not with the dopant or any sample from the GC column. This prevents burning of any compound which might create soot or otherwise form an undesired deposit on the interior of the ECD equipment. Two concentrically positioned tubes are introduced into the ECD equipment. They are inserted into the flowing stream of helium gas which sweeps the area where the spark is formed. In routine operation, the helium flow is typically in the range of about 20-150 milliliters or so. A typical flow is about 100 milliliters. This flow enables the insertion of two concentrically located tubes downstream which introduce additional flow but which gases cannot migrate against the larger helium flow which is significantly larger, perhaps 5 to 50 times larger in volume. One injection tube which is positioned in the ECD chamber delivers a trace or dopant gas. It is a gas which is readily ionized and which interacts with the photon ionization emission from the spark discharge. One example of the trace gas introduced is hydrogen. It interacts readily and is highly mobile, diffusing in the region downstream from the point of introduction. A second tube is utilized to inject an additional flow downstream. The second tube is located so that its discharge is into the hydrogen diffused area so that the GC column effluent is introduced. The GC column effluent is primarily a carrier gas which is neutral electrically. The second tube also discharges the separated peaks from the GC column. Because they are not electrically neutral, they create a current flow variation in comparison with the current flow established in quiescent conditions when only the trace or dopant gas is introduced to the ECD. For that reason, a steady state condition is first established. This occurs when a specified flow rate of the dopant gas is introduced. An example might be an introduction rate of 1 milliliter per minute of hydrogen which is introduced into a flow of 100 milliliters per minute of helium. The discharge from the GC column typically will be something of the same magnitude, perhaps a fraction up to about 2 or 3 milliliters per minute. This is introduced downstream of the dopant introduction point. If the dopant is hydrogen (the most mobile of molecules), then interaction is readily obtained because the hydrogen will diffuse easily through the flowing stream of helium. The hydrogen flow in the quiescent state establishes a current flow which is scaled to a maximum value. Thereafter when a peak is separated by the GC column, the peak constituent delivered into the ECD causes a drop in current. This enables measurement of peak amplitude. As will be understood in the detailed description of current flow in the following paragraph, the peak amplitude creates a current flow drop in the negative direction. Helium ions and free electrons are created when the preferred helium gas flow is exposed to the spark creating electrodes through the reaction EQU He=He.sup.+ +e.sup.- ( 1)
where He.sup.+ denotes a positively charged He ion and e.sup.- denotes a free electron. After the spark discharge, some of the energy from the free electron flux interacts with neutral helium in the flow gas to form excited helium He* through the reaction EQU e.sup.- +He=He*+e.sup.- ( 2)
Subsequently, He* decays by the emission of a photon through the reaction EQU He*=He+photon (3)
As dopant gas such as hydrogen is introduced into the chamber, the photons produced by the reaction of Equation (3) interact with the hydrogen to produce H ions and free electrons through the reaction EQU photon+H=H.sup.+ +e.sup.- ( 4)
The flow of hydrogen ions and free electrons establish the previously defined base line current of the device. Once an "unknown" compound, denoted generically as "AB", is introduced into the system, the free electrons of Equation (4) can initiate several classes of reactions including EQU e.sup.- +AB=(AB)*+e.sup.- ( 5)
and EQU (AB)*=AB+photon (6)
where (AB)* denotes an excited state of compound AB. Upon introduction of an unknown compound AB, the reaction of Equation (6) competes for some of the electron population under steady state or baseline conditions. As a result, the introduction of a compound AB results in an observed current flow-drop in the ECD system.
Summarizing, the present ECD system utilizes the spark formed across a pair of terminals transverse to the helium flow introduced into the system. Radiation resultant from the photon ionization of the helium creates an interaction downstream with an introduced dopant in small volume. This establishes a first current rate which is the steady state condition for the ECD. Downstream of the dopant introduction point, another introduction point is used to inject the GC column eluted peaks along with the GC column carrier. The first, second and third terminals are utilized having the preferred form of encircling rings about the passage, and an electrometer output provides measurements of the current flow from the ECD. The output signal is typically recorded by a time based recorder.